1. Field of the Invention
The present invention relates to an ultrasonic object detecting apparatus for detecting the presence of a human body, vehicle and the like by using the reflection of ultrasonic waves.
2. Description of the Related Art
FIG. 5 shows a block diagram of a conventional ultrasonic object detecting apparatus. In FIG. 5, an oscillator 1 is connected with a tone burst generating circuit 2 and a gate pulse circuit 3. The tone burst generating circuit 2 is connected with an ultrasonic wave transmitter 5 via a transmission and amplification circuit 4 for forming a transmission circuit. Similarly, an ultrasonic wave sensor 6, a reception and amplification circuit 7, a detector 8 and a discriminator 9 are coupled with each other as shown in the drawing for forming a reception circuit.
Reference timing signals are generated by the oscillator 1 and radio frequency pulses each having a width of about 10 ms are generated at intervals of usually 200 ms by the tone burst generating circuit 2 and ultrasonic wave pulses are emitted from the ultrasonic wave transmitter 5 via the transmission and amplification circuit 4. If an object exists in front of the ultrasonic wave transmitter 5, a wave reflected from the object is received and amplified by the ultrasonic wave sensor 6 and the reception and amplification circuit 7 and is converted into a direct current level by the detector 8. When the reflected wave, which has been converted into the direct current level by the detector 8, is received within a period of the gate pulse outputted by the gate pulse circuit 3, which generates gate pulses representative of a period of time during which the reflected wave which is shifted by a given real period of time from the pulses generated by the tone burst generating circuit 2 may be detected, a detection signal is outputted by the discriminator 9.
However, the conventional art having such a structure has a drawback as follows. Since the level of the reflected wave detected by the ultrasonic object detecting apparatus changes greatly due to the difference in distance between the object and the ultrasonic object detecting apparatus as shown in FIG. 6a, it is necessary to increase the gain of the reception and amplification circuit 7 so that an object which is far from the detecting apparatus can be detected. If the gain is excessively increased, the level of the reflected wave from the surface of a close floor or a wall which is located out of a detecting area would be increased. Thus only the reflected wave from the floor surface is detected as shown in FIG. 6c during the gate period as shown in FIG. 6b, and misdetection takes place. Accordingly, it is difficult to stably detect the presence of an object over a continuous range from a short distance such as several centimeters to a long distance such as about ten meters.